1. Technical Field
The present invention relates in general to forming metal alloys and, in particular, to a method for gas-phase alloying of metallic materials with timed deposition processing.
2. Description of the Related Art
Many metal objects are produced by thermomechanical processes including casting, rolling, stamping, forging, extrusion, machining, and joining operations. Multiple steps are required to produce a finished article. These conventional operations often require the use of heavy equipment, molds, tools, dies, etc. For example, a typical process sequence required to form a small cylindrical pressure vessel might include casting an ingot, heat treating and working the casting to homogenize it by forging, extrusion, or both, machining a hollow cylinder and separate end caps from the worked ingot and, finally, welding the end caps to the cylinder.
Conventional production methods are subtractive in nature in that material is removed from a starting block of material to produce a more complex shape. Subtractive machining methods are deficient in many respects. Large portions of the starting material are reduced to waste in the form of metal cuttings and the like. These methods also produce waste materials such as oils and solvents that must be further processed for purposes of reuse or disposal. Even the articles produced are contaminated with cutting fluids and metal chips. The production of such articles also requires cutting tools, which wear and must be periodically reconditioned and ultimately replaced. Moreover, fixtures for use in manufacturing must be designed, fabricated, and manipulated during production.
Machining is even more difficult when a part has an unusual shape or has internal features. Choosing the most appropriate machining operations and the sequence of such operations requires a high degree of experience. A number of different machines are needed to provide capability to perform the variety of operations, which are often required to produce a single article. In addition, sophisticated machine tools require a significant capital investment and occupy a large amount of space. In contrast, using the present invention instead of subtractive machining provides improved solutions to these issues and overcomes many disadvantages.
Another difficulty with conventional machining techniques is that many objects must be produced by machining a number of parts and then joining them together. Separately producing parts and then joining them requires close-tolerance machining of the complementary parts, provision of fastening means (e.g., threaded connections) and welding components together. These operations involve a significant portion of the cost of producing an article as they require time for design and production as well as apparatus for performing them.
Titanium has been used extensively in aerospace and other manufacturing applications due to its high strength-to-weight ratio. To increase the usefulness of titanium, various titanium alloys have been produced, many being tailored to provide desired characteristics. However, the equilibrium solute levels (as measured in weight-percent) in conventionally processed titanium alloys are below that which maximizes the beneficial effect of the solute.
For example, in concentrations over 500 ppm, nitrogen is typically considered a contaminant in titanium alloys. At levels higher than 500 ppm, the tensile strength increases greatly with a corresponding drop in tensile ductility. Additionally, solidification cracking can be a serious problem at high nitrogen levels. It is this embrittling effect that prohibits the use of nitrogen as a significant alloying agent.
Titanium alloys typically exhibit low wear resistance due to their low hardness. Under certain circumstances, titanium also can be subject to chemical corrosion and/or thermal oxidation. Prior art methods for increasing the hardness of titanium alloys have been limited to surface modification techniques. For example, a hard face coating is a discrete surface layer applied to a substrate and is subject to delamination. Current methods are also subject to macro and micro cracking of the surface-hardened layer. For example, U.S. Pat. Nos. 5,252,150 and 5,152,960 disclose titanium-aluminum-nitrogen alloys. These patents disclose an alloy that is formed through a solid-state reaction of titanium in a heated nitrogen atmosphere. The alloy is formed in a melt with aluminum to create the final alloy product.
Rapid solidification processes (RSP) also can be used to increase the amount of solute levels in alloys. In these processes, a rapid quenching is used in freezing the alloy from a molten state so that the solutes remain in desired phases. After quenching, diffusion may allow for dispersion throughout the material and agglomeration at nucleation sites, which further improves the desired characteristics of the alloy.
Creating metallic matrix composite structures with RSP is a difficult procedure that usually takes multiple complicated steps. Differences in material properties (e.g., yield strength, coefficient of thermal expansion, etc.) can make it very difficult to form a solid, fully dense structure without serious stress concentrations that can lead to cracking. While this type of process is used to form products such as powder, flake, or ribbon forms, it is unsuitable for manufacturing applications requiring material in bulk form. Thus, an improved metal alloy and process for producing the same would be desirable for many practical applications.